1. Field of the Invention
The invention relates to mounting gas turbine engines turbine nozzle segments and, particularly, for cantilevered mounting of such segments.
2. Description of Related Art
In a typical gas turbine engine, air is compressed in a compressor and mixed with fuel and ignited in a combustor for generating hot combustion gases. The gases flow downstream through a high pressure turbine (HPT) having one or more stages including one or more HPT turbine nozzles and rows of HPT rotor blades. The gases then flow to a low pressure turbine (LPT) which typically includes multi-stages with respective LPT turbine nozzles and LPT rotor blades.
The HPT turbine nozzle includes a plurality of circumferentially spaced apart stationary hollow nozzle vanes supported between radially outer and inner bands. Typically, a single chamber impingement baffle is inserted in each hollow airfoil to supply cooling air to the airfoil. Each baffle can be fed through a single spoolie located radially outwardly of the outer band of the nozzle.
The turbine rotor stage includes a plurality of circumferentially spaced apart rotor blades extending radially outwardly from a rotor disk which carries torque developed during operation. The HPT nozzles are typically formed in arcuate segments having two or more hollow vanes joined between corresponding segments of the outer and inner bands. Each nozzle segment is typically supported at its radially outer end by a flange bolted to an annular outer casing. Each vane has a cooled hollow airfoil disposed between radially inner and outer band panels which form the inner and outer bands. The airfoil, inner and outer band portions, flange portion, and intake duct are typically cast together such that each vane is a single casting. The vanes are brazed together along interfaces of the flange segments, inner band panels, and outer band panels to form the nozzle segment. Two or more airfoils may also be cast together in a single vane or nozzle segment.
Certain two-stage turbines have a cantilevered second stage nozzle mounted and cantilevered from the outer band. There is little or no access between first and second stage rotor disks to secure the segment at the inner band. Typical second stage nozzles are configured with multiple airfoil or vane segments. Two vane designs, referred to as doublets, are a very common design. Doublets offer performance advantages in reducing split-line leakage flow between vane segments. However, the longer chord length of the outer band and mounting structure compromises the durability of the doublet. The longer chord length causes an increase of chording stresses due to the temperature gradient through the band and increased non-uniformity of airfoil stresses. The box structure of a vane doublet also contributes to uneven stresses in the segment. The trailing vane of a doublet typically sees significantly higher stresses which limits the life of the segment.
It is highly desirable to have a turbine nozzle segment that provides access for a cooling air supply between first and second stage rotor disks and the ability to be cantilever mounted from the outer band. It is also desirable to have turbine nozzle segments that avoid reduction in the durability of multiple vane segments due to longer chord length of the outer band and mounting structure. It is also desirable to have turbine nozzle segments that avoid increase of chording stresses due to temperature gradient through the band and increased non-uniformity of airfoil stresses due to longer chord length of the multiple vane segments. It is also desirable to have turbine nozzle segments that avoid increase of stresses in a trailing vane of a doublet or other multiple vane segment which limits the life of the segment.
A single vane segment, referred to as a singlet, is disclosed in U.S. patent application Ser. No. 10/375,585 and has a bifurcating stiffening rib which divides a bifurcated cavity into forward and aft cavities requiring two baffles. A bifurcated impingement is disclosed in U.S. patent application Ser. No. 10/375,415. Due to the cantilevered design of the second stage nozzle, there is insufficient access between the first and second stage rotor disks to feed cooling air from a radially inner side of the nozzle. Thus, both baffles must be fed cooling air from the outer side of the nozzle. U.S. patent application No. identified as Docket No. 131816 discloses a bifurcate impingement baffle for the bifurcated cavity that is able to be feed cooling air from the outer side of the nozzle to both the forward and aft cavities.
A nozzle singlet offers significant durability improvements compared to more common doublet configurations. A high twist angle of the airfoil causes the resultant gas load vector to fall off a wheelbase of the outer band. The wheelbase is an arc of an aft end of the outer band. This induces the nozzle segment to rotate about a radial line normal to a centerline of the engine and turbine creating a significant challenge to the mounting and sealing of a singlet as well as other cantilevered nozzle segments, particularly, those having a resultant gas load vector that falls off a wheelbase of the outer band.